The invention relates to a laser with a housing such as at least one laser tube for a laser gas or a mixture of laser gases, with an electrode arrangement that comprises at least two electrodes, with a voltage supply unit by means of which a voltage can be fed to electrodes of the electrode arrangement in such a way that a discharge area forms in the laser gas or laser gas mixture, in which the laser gas or laser gas mixture is excited, and with a system for modifying the distribution of the intensity of the laser light over the laser beam cross-section (mode).
Lasers with a housing, an electrode arrangement and a voltage supply unit of the type described above are described in U.S. Pat. No. 4,757,511, for example. From this publicationxe2x80x94which is moreover expressly referred to for explaining all details not described more thoroughly herexe2x80x94lasers are known in which the laser gas is a mixture of a laser-active gasesxe2x80x94particularly CO or CO2xe2x80x94and a few other components, e.g. N2 and helium. Such lasers are used, among other things, for the processingxe2x80x94i.e., cutting or weldingxe2x80x94of workpieces. In this connection, the voltage fed to the electrodes of the electrode arrangement can be a direct or alternating voltage. A high-frequency alternating voltage (HF voltage) is preferred. In the process, the electrodes of the electrode arrangement can be arranged on or in the housing.
In the case of the types of lasers described in U.S. Pat. No. 4,757,511, individual electrode pairs are twisted against each other in order to obtainxe2x80x94xe2x80x9cintegratedxe2x80x9d over the length of the laserxe2x80x94as uniform a discharge as possible.
When processing workpieces, factors that play a role are, principally the output of the laser and the beam diameter, which depend on the geometry of the laser tube and the electrode arrangement and on the respective output, and the distribution of the intensity of the laser light over the cross-section of the unfocussed laser beam (also referred to as xe2x80x9cmodexe2x80x9d). The width of a cutting clearance, or kerf or of a welding seam, is determined by the beam diameter and the aforementioned intensity distribution. The thickness of material able to be cut as well as the attainable cutting speed are likewise dependent upon the two aforementioned parameters. Furthermore, beam diameter and mode are decisive for the thermal load of the optic elements used for beam shaping and beam guidance.
A laser beam with a high intensity maximum in the middle, from which the intensity decreases all around (also referred to as xe2x80x9cGauss modexe2x80x9d), can very well be focused on a small spot, thus resulting in a very high energy density. High cutting speeds can thereby be achieved. In addition, very narrow cutting clearances are obtained, which is desirable when cutting thin metal sheets. On the other hand, however, with the Gaussian mode the optic elements used for beam formation and beam guidance are under considerable thermal load precisely at high outputs due to the intensity maximum.
When cutting thick metal sheets, more than a high laser output is necessary. In addition, a certain minimum width of the cutting gap or kerf is desirable for blowing out the slag. These requirements can be best fulfilled by a laser beam in which the intensity maximum of the unfocused beam is situated not in the middle but rather at the periphery (so-called xe2x80x9cring modexe2x80x9d). In the case of the ring mode, even at high laser outputs, the optic elements of the concerned laser are less burdened than with the Gaussian mode because the energy density is lower.
In welding, the required laser output is likewise quite high, while the demands on the laser beam""s ability to focus are comparably slight because no cutting clearance needs to be produced. In welding, the work is also typically done with a laser beam that has no pronounced intensity maximum in the middle. On the other hand, with other processing a nearly rectangular intensity distribution (so-called xe2x80x9cflat top modexe2x80x9d) can be useful.
Under the circumstances described in the preceding discussion, it is particularly advantageous if the distribution of the intensity of the laser light over the cross-section of the unfocused laser beam, i.e. the mode, can be adapted to the processing method carried out in each case.
With most currently known lasers, the mode is determined by the unalterable geometry of the laser tube, the arrangement of the electrodes and the mirrors, by the feed of energy and the properties of the lasing gas. Practically speaking, this predetermined mode cannot be influenced by the laser user. Setting the distribution of the intensity of the laser light over the cross-section of the laser beam to adapt to the respective case of application is not easily possible.
For this reason, it was attempted in the past to achieve an adaptation of the laser mode to the respective processing case by replacing essential components or entire laser systems or machines (cutting lasersxe2x80x94welding lasers). This method of procedure requires not only high investment costs but also makes a quick retooling for different processing methods impossible.
A laser of the kind indicated hereinbefore, i.e. a laser with a system for modifying the distribution of the intensity of the laser light over the laser beam cross-section, is known from German Patent No. 44 01 597. However, this publication leaves unresolved the question of what technical measures are used to bring about the modification of the mode.
It is an object of the present invention is to provide a novel laser of the kind hereinbefore described which permits facile modification of the distribution of the intensity of the laser light over the laser beam cross-section, i.e., a modification of the mode, to adapt the intensity distribution or mode to the desired application with low structural/equipment expenditure.
Another object is to provide such an adjustable laser with relatively low additional cost and relatively little additional components.
It has now been found that this technical problem may be solved according to the present invention by incorporating in a gas laser a system for modifying the mode which comprises means for adjusting the amplification profile of the laser over the excitation cross-section of the housing for the laser gas or the laser gas mixture. The excitation cross-section in the aforementioned sense refers to a cross-section perpendicular to the laser beam propagation direction, in which the excitation of the laser gas or laser gas mixture takes place. There is a different amplification in each point of an excitation cross-section. The spatial distribution of these amplification points is referred to as the amplification profile which is in turn decisive for the mode being set. In that the present invention applies to mode modification on the amplification profile of a laser, it differs from xe2x80x9cmode diaphragmsxe2x80x9d of the known kind, for example. Such mode diaphragms are aperture diaphragms used to bring about a damping of modes that were first set. As will be described in detail below, the mode can be set according to the invention by means of systems that are present on lasers anyway or that can be provided in such lasers without great expense.
In one embodiment the amplification profile of the laser may be adjusted over the excitation cross-section of the housing for the laser gas or the laser gas mixture by including a device by means of which the excitation potential fed by the electrodes into the laser gas or the laser gas mixture is able to be modified. By setting the supplied excitation potential at relatively high values, the light amplification in the area of the beam axis can be minimized. The result is then a ring mode. This is conveniently effected by varying the amplitude of the HF voltage fed to the electrodes or by modifying the keying into of the pulsing excitation.
Generally, the excitation potential fed into the laser gas is variable from 0 to KW and ever higher. In one embodiment, several predetermined or fixed values or ranges for the excitation potential can be incorporated in the laser controls. Preferably at least one fixed value or range is above 2 KW. By varying the excitation potential, the mode is continuously adjustable.
In another embodiment, numerous measures are provided alternatively or jointly; the objective of all of them is, by influencing physical properties of the laser gas or the laser gas mixture, to locally adjust the resulting light amplification to thereby design the amplification profile of the laser on the whole. These measures also make it possible, starting from a Gauss mode, to minimize the light amplification in the area of the beam axis and to thereby produce a ring mode. This can be achieved, for example, by setting the temperature of the laser gas or the laser gas mixture at relatively high values and/or by targeted modification of the partial pressures of the components of the laser gas or the laser gas mixture and/or by setting a relatively low flow rate of the laser gas or the laser gas mixture in the corresponding area.
When switching on the voltage at the electrodes, as a rule the discharge of the laser gas or the laser gas mixture does not xe2x80x9cignitexe2x80x9d uniformly over the entire surface of the electrodes. Rather, a discharge channel first forms between two relatively small areas on the surfaces of the mutually associated electrodes. The electric potentials between the individual areas of the mutually assigned electrodes determine the shape and the position of the resulting xe2x80x9cignition discharge channelxe2x80x9d. The design of the electrodes thus essentially predetermines the shape and the position of the channel in which the discharge ignites.
The discharge starts in the area in which the field strength is greatest. Afterward, the discharge channel widens toward the sides (relative to the discharge channel), in such a way that between areas of mutually assigned electrodes a discharge also xe2x80x9cfiresxe2x80x9d for which the energy conditions are less favorable than for the xe2x80x9cstart areaxe2x80x9d, that is, for the area in which the discharge first ignited.
In this connection, the rate at which the ignition discharge channel spreads out over the entire cross-sectionxe2x80x94cross-section possible based on the electrode designxe2x80x94of the housing for the laser gas or the laser gas mixture, particularly over the possible cross-section of the laser tube, depends essentially on the geometry of the electrodes and the properties of the laser gas.
If the laser is operated in key operation, i.e. with pulsing excitation, the voltage required for the discharge of the laser gas or the laser gas mixture is not constantly connected at the concerned electrodes of the electrode arrangement. Rather, the electrode voltage is switched on and off The frequency of the ON/OFF change is then referred to as the xe2x80x9ckeying frequencyxe2x80x9d. xe2x80x9cKeying frequencyxe2x80x9d is understood to mean the relation of two consecutive switched-on periods on the one hand and the switched-off period for the electrode voltage between these switched-on periods. On a keyed laser the switched-on period determines the shape and position of the discharge forming in each cross-section between the respective electrodes (time-dependent). The shape/the position of the discharge channel between the concerned electrodes in turn defines the density distribution of the laser-active particles of the laser gas or the laser gas mixture in the respective cross-section of the housing for the laser gas or the laser gas mixture or the amplification profile of the laser.
According to the invention, the shape of the electrode surfaces facing each other is therefore designed in such a way that, by selecting different keying frequencies or types of keying operation, different shapes of discharge areas in the cross-section of the housing for the laser gas or the laser gas mixture and thereby different modes can be set in targeted manner.
If, for example, a laser has one or more laser tubes with electrodes at which the discharge starts in the electrode peripheral areas, at high keying frequencies this results in a peripherally accentuated amplification profile, because almost no discharge takes place in the middle of the laser tube. If several pairs of electrodes twisted against each other are arranged along the laser tube or laser tubes, the peripherally accentuated amplification profiles add up in such a way that the laser operates in a (nearly rotationally symmetrical) ring mode. If the keying frequency is then reduced, a noteworthy discharge also takes place in the center of the laser tube. With an appropriately selected keying frequency, a Gauss mode or a xe2x80x9cflat-top modexe2x80x9d then sets in.
Accordingly, in a laser the electrodes of which are such that the discharge begins from the middle of the electrodes, at high keying frequencies the result is a mode accentuated in the middle. If several pairs of electrodes twisted against each other are arranged consecutively along the laser tube or laser tubes, a Gauss mode sets in, i.e., a (nearly) rotationally symmetrical intensity distribution with a maximum on the optic axis of the laser and a steep drop toward the periphery. By reducing the keying frequency or by changing the keying ratio in the sense of a prolongation of the period during which a voltage is supplied at the electrodes of the electrode arrangement, the discharge area in the cross-section of the housing for the laser gas or the laser gas mixture is enlarged. The mid-section accentuation of the intensity of the laser light over the laser beam cross-section weakens compared to previously prevailing conditions.
When using electrodes whose surfaces facing the laser gas or the laser gas mixture have several differently designed areas, by modifying the keying ratio, that is, the relation of the switched-on and switched-off periods of the concerned electrodes of the electrode arrangement, it is possible to switch between Gauss mode and ring mode and, should the occasion arise, other modes such as the xe2x80x9cflat-top mode. It should be expressly clarified at this point that by implementing the basic theory according to the invention, it is not only possible to switch the mode of a laser to the aforementioned two or three types of modes but it is also possible to produce other modes adapted to the respective processing case, e.g. modes with an asymmetric intensity distribution. This is possible, for example, by combining differently shaped electrodes and different rotational angles of individual electrode pairs along the laser.
In addition, the twisting of individual electrode pairs against each other can be changed between consecutive processing steps.
In this connection, the keying frequency can range from 0 Hz (no keying, i.e. continuous wave or cw operation ) to more than 100 kHz (the frequency indication relates to the number of keyings, i.e., the number of switch-on steps per second).
It is furthermore advantageous if the keying ratio (pulse length or switched-on period per keying/pause length, i.e., switched-off period between two consecutive keyings) is variable. What is decisive is the period during which the voltage is connected. A measure for this is the keying frequency as well as the keying ratio.
In a further preferred design of the invention, it is provided for that on a control unit for the laser, several fixed keying frequencies can be set each of which brings about a specific intensity distribution or a specific mode. Of course, it is also possible to provide for a continuous adjustability of the keying frequency and/or the keying ratio.
With typical gas lasers, such as CO2 or CO lasers for processing work pieces, at least one keying frequency is preferably greater than 10 kHz. In this connection, the keying frequency may be greater than 50 kHz, particularly greater than 80 kHz.
In one embodiment, switching elements each connect one or more electrodes of the electrode arrangement to the voltage supply unit. Depending on which of the various electrodes of the electrode arrangement are connected to the voltage supply unit, different discharge areas or channels ensue in the laser gas or the laser gas mixture over the cross-section of the housing for the laser gas or the laser gas mixture.
The shape and/or the position of the discharge area between two or more activated electrodes, i.e. electrodes connected with the voltage supply unit, determines the amplification profile of the laser and thus the intensity distribution of the laser light over the laser beam cross-section.
If the laser is then provided with numerous electrodes assigned or assignable to each other and able to be arbitrarily activated, by selecting the activated electrodes, i.e., by selecting the electrodes between which a discharge path builds up, the amplification profile can be set differently without needing to modify the geometry of the housing for the laser gas or the laser gas mixture and/or the optic components and/or the electrodes of the electrode arrangement of the laser or to replace parts.
To form a specific amplification profile and thus a specific mode, a control unit appropriately controls the switching elements.
In this connection, the electrodesxe2x80x94at least a portion of which can be arbitrarily activatedxe2x80x94can be arranged at a distance from each other in the same cross-section of the housing for the laser gas or the laser gas mixture or in the direction of the laser beam or the housing""s longitudinal axis. The latter solution has the particular advantage that the individual excitation profiles then do not overlap and thereby disturb each other, and that by xe2x80x9cadding upxe2x80x9d the amplification profiles present in the individual cross-sections along the housing, almost any desired intensity distributions can be achieved, but particularly the frequently required rotationally symmetrical distributions such as Gauss or ring mode.
Elements that have an amplification or weakening function can be used as switching elements. Simple on/off switches that connect the voltage at specific electrodes or switch specific electrodes off are particularly preferred as switching elements.
If the impedance changes when switching between various electrodes, an impedance compensation is preferably provided so that comparable conditions always ensue.
As already explained, in addition to the choice of keying frequency and keying ratio, the design of the electrode surfaces facing the laser gas, for example, is also essential for the invention.
Electrodes of the widest variety of arrangements or designs can be used on lasers according to the invention. Thus, the electrodes of the electrode arrangement may be arranged shifted in the direction of the periphery of the laser beam, they may have a different bend and/or a different azimuthal span. Due to the overlappingxe2x80x94connected with the shifting of the electrodesxe2x80x94of the discharge channels between the individual electrodes, practically any desired mode can then be generated. In this connection, the individual electrodes may also be at a distance from each other in azimuthal direction.
By means of the azimuthal span of the electrode surfaces, for an ignition in the middle the discharge channel width ensuing at low keying frequencies is determined, or when igniting from the periphery, the diameter of the ring is determined in the case of a ring mode.
Within the framework of the invention, electrode surfaces are conceivable that essentially follow the course of the surface of the housing for the laser gas and laser gas mixture, that is, the wall of a round or polygonal laser tube, for example.
In addition, the electrode surfaces can have in the central area at least one raised area, such as a peak or edge, or at least one recess, such as a groove, for example. In the case of a rise in the central area, the discharge starts from the raised area after switching on the voltage, while in the case of a recess, the discharge ignites from the areas surrounding it.
In this connection, the raised areas can be formed by projections, peaks or a ridge design of the respective electrode surface. Useful width or depth relations on electrodes provided with a recess are described hereinafter.
Generally speaking, the electrodes do not all have to be designed identically. For example, they may have differently designed surfaces on their sides facing the laser gas or the laser gas mixture. It is advantageous, however, if the design of the surfaces facing the laser gas is identical for electrodes assigned to each other in pairs, because at ignition, a defined discharge path with of two-fold symmetry then ensues whichxe2x80x94as will be explained belowxe2x80x94simplifies the setting of a rotationally symmetrical intensity distribution.
In addition, the electrodes of the electrode arrangement can be at least partially at a distance from the axis of the laser beam.
Furthermore, it is possible for the surfaces facing the laser gas of at least individual electrodes to consist of several parts designed differently. By means of the control unit, the individual parts can then be connected separately. In this regard, the distance of the individual parts from the axis of the laser beam and/or the azimuthal span of the individual parts can be different. In particular, it is possible for the electrodes to have raised areas or drops such as recesses or grooves, etc. The discharge is then of different xe2x80x9cintensityxe2x80x9d depending on the distance between the electrode partial surfaces assigned to each other.
In principle, the invention can be implemented in the case of all voltage-excited lasers. A particularly preferred case, however, is the one in which the voltage supply unit feeds an alternating voltage to the electrodes and the frequency of this alternating voltage is greater than 100 kHz and, in particular, between 1 MHz and 10 MHz (high-frequency excited laser). One should distinguish between the frequency of the voltage fed to the electrodes and the aforementioned keying frequency, i.e., the frequency with which the concerned electrodes of the electrode arrangement are connected to the voltage or separated from it.
The invention is explained below in more detail using schematic diagrams of forms of construction. The drawings are expressly referred to regarding the disclosure of all details not explained more thoroughly in the text. The following are shown: